Press Fit Nylon Seal?

I am the defacto primary gunsmith for the MIT collegiate pistol team.
They have a large number of Czech CO2 pistols that were made about 20 or
more years ago. They shoot very accurately, and are generally pretty
reliable. The primary firing seal is dying on a few of them, and the
replacements we have don't fit due to a design change at some point.
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The seal is a short hard white plastic tube, press fit into a hole in the
pistol frame. The actual firing seal is formed with a conical pin that
presses into the hole through the center of the plastic seal. On the
theory that the most likely seal material is Nylon 6/6, I bought some rod
stock, and I'm going to try making new seals from scratch.
The recommended approach for removing the old seals it to run a brass
wood screw into the seal & then pull it out. The pulling part takes
quite a bit of force, and the process means the seal can't be re-used
once removed. However, the one I extracted is sufficiently intact to
take some measurements from. Give or take a few tenths, the seal
measures 0.3157" OD. The bore in the pistol measures 0.3138" ID.
I've done some reading on both Nylon & press fitting plastics, and there
are several issues that complicate matters:
1) Nylon is hygroscopic, and will swell depending on the moisture
content. That means if I machine one now when it is humid, it will
shrink in the winter. On the otherhand, machining it will heat it a bit
& dry it out slightly.
2) Nylon has a pretty high thermal coefficient of expansion. That means
I probably need to let it cool off to measure it when I'm machining it.
They recommend chilling the part to shrink it for the press fitting, but
with high humidity, if I get it at all cool, it will be covered with
moisture (if not frost). I'd have to drop the temperature ~ 60 degrees F
to shrink the part by 1 mil, which definitely gets it into the frost
category.
3) If I make it sufficiently oversized to make sure that moisture &
thermal variations don't allow it to shrink enough to be a problem, it
can creep over time & loosen up. I suspect that may be why some of them
are failing after all this time, possibly shrinking enough to allow CO2
to sneak past them. There's a good web site that talks about this here:
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This also means that the measured size of the seal I removed is probably
smaller than it was originally cut. As extracted, they had about a 2 mil
interference fit, which would seem to be plenty. I need to double check
the calculations, but that should result in over 1500 psi of expansive
force between the seal & the bore of the hole. It's also enough to
produce significant creep over time:
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1% is over 3 mils, although as it creeps, the forces will drop, and
presumably it will reach something like steady state when the force drops
enough.
4) Annealing: Lots of folks recommend annealing Nylon before trying to
machine it. From some DuPont literature, 3/8" rod stock should only
require an hour at a couple hundred F. They recommend not doing it in
air, and suggest heating it in wax or oil, and then cooling it slowly &
evenly to avoid surface stresses. I'm thinking heating it in cooking oil
with a thermometer, and then letting the whole mess cool slowly.
There are other plastics that have lower moisture issues than Nylon 6/6,
but I think the TCE & creep is going to be similar for a lot of them.
I seem to have opened a serious can of worms here. I'm thinking about
just going for it, cutting one 2.5 mils oversized without annealing. If
anyone has any real experience to help sort through the chaos, I'm all
ears.
Thanks!
Doug White
Reply to
Doug White
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Try pulling the seal with a brass screw fitted in the head of a small slide hammer. Available at Harbor Freight for less than $15.
Reply to
azjohn
It could be Acetal.
I recently made a replacement lighter plug tip retainer from a Nylon spacer and some larger Acetal rod. Both machined nicely to intended size with a good finish, including on the single-pointed threads.
Before someone asks why bother, it's on an Auto-Air laptop charger and the Air jack is in the back end of the Auto plug.
jsw
Reply to
Jim Wilkins
Page 19 of this guide has some guidelines for press fitting for various filled nylons and other plastics as well as the effets of water absorption. They seem to indicate it is mainly an issue in wet environments.
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If wear is an issue, you might want to look into one of the filled nylons.
Reply to
anorton
Have you contacted Beeman? They used to sell Czech air guns and may still do. They have a large repair dept which may even have the part you need.
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Reply to
Artemus
I would het the nylon to be sure the moisture is out of it, then machine it over sized enough to counter the effects of the CO2 on the seal. If you're really worried you could contact Crosman and talk to them, They use plastic valve seats in most of their items and might be able to tell you what you need to know. 800-724-7486
Reply to
Steve W.
azjohn wrote in news:jt7pfo$4jo$ snipped-for-privacy@wieslauf.sub.de:
If the re-sealing is a success, I will get more serious about tooling up to fix more in the future. I had envisioned a threaded tool where you screw the brass wood screw into the seal, and then run a nut down a threaded piece to extract the old seal.
Doug White
Reply to
Doug White
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Reply to
Steve W.
"Artemus" wrote in news:jt7vcv$6k1$ snipped-for-privacy@dont-email.me:
I've tried a number of places, and the few that have seals don't have the size I need. So far we've identified at least 3 design variations with different sized seals.
I checked Beeman's site. As far as I know, they never sold the Tau-7's, and they don't list them as a make/model that they service.
Doug White
Reply to
Doug White
"Jim Wilkins" wrote in news:jt7qa0$8ll$1@dont- email.me:
I thought about that. I went with nylon because it is more impact resistant, and the metal firing valve whacks into it with every shot. I also figured it's a more likely material for a non-high-end commercial product. Now that I have all the measurements I need off the original, I can try to burn a sliver & see if it burns & smells like nylon or not. There's a useful chart here:
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but I also have known samples of nylon 6/6 & acetal to compare with.
Doug White
Reply to
Doug White
If I had to design it, I'd use Delrin, not nylon.
Doug White wrote:
Reply to
Jim Stewart
"Steve W." wrote in news:jt7vqo$8bm$ snipped-for-privacy@dont-email.me:
Neither one lists any Czech air pistols, although JG seems to cover enough bases that I'll send the an email, just in case.
Thanks!
Doug White
Reply to
Doug White
Gunner Asch wrote in news: snipped-for-privacy@4ax.com:
Do you mean HDPE? I thought about UHMW PE. All three materials are much softer than the original seal. I'm concerned with cold flow. The valve seat is under considerable pressure, and a soft material could mush down & cause problems. They can also be even nastier to machine than nylon. I'm fighting with some UHMW on another project right now.
Doug White
Reply to
Doug White
Yes, but the worst that can happen if you call them is they'll say "no" and hang up. At best you'll get the parts you need, so unless you're looking forward to making these critters, it doesn't hurt.
Reply to
Tim Wescott
You might want to do a flame test.
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Reply to
PrecisionmachinisT
I used to work in a semiconductor plant that used a lot of Nupro Valves, they all had Kel-F stem tips, very hard plastic but with chemical properties of Teflon. Most likely very expensive.
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No idea what it machines like.
ebay item Item number: 230806546854 1/2" rod 12" long
or search ebay for kel-f or pctfe
this might be the high tech replacement for whatever was there.
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Reply to
Randy
My gut reaction is to scare up a native speaker of Czech and call the manufacturer.
Reply to
J. Clarke
The manufacturer will have someone who understands English, even if they can't quite speak it over the phone. So send them a letter first. When writing the letter, stick to short simple sentences and no slang.
Joe Gwinn
Reply to
Joseph Gwinn
That's another option, but for someone who is on staff at MIT it seems to me that calling admissions and asking if there's a Czech enrolled would be a fairly simple first step.
Reply to
J. Clarke
Ahh. Not a widely-available option, but it ought to work.
Joe Gwinn
Reply to
Joseph Gwinn

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